Base station which uses pre-rotation before transmission

ABSTRACT

The present invention is an apparatus for reducing transmission errors by a base station when transmitting a communication signal. An RF communication signal is received at the base station and analyzed for errors. A correction signal is then generated based upon the analysis. Prior to transmission of the communication signal from the base station, the communication signal is corrected using the correction signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/820,014,filed on Mar. 28, 2001; now pending which claims priority fromProvisional Application No. 60/192,670, filed on Mar. 28, 2000.

BACKGROUND

The present invention relates generally to digital communications. Morespecifically, the invention relates to a system and method forpre-rotating a digital spread spectrum signal prior to transmission inorder to improve receiver accuracy and recovery of the phase andfrequency information by the receiver.

Many current communication systems use digital spread spectrummodulation or code divisional multiple access (CDMA) technology. Digitalspread spectrum is a communication technique in which data istransmitted with a broadened band (spread spectrum) by modulating thedata to be transmitted with a pseudo-noise signal. CDMA can transmitdata without being affected by signal distortion or an interferingfrequency in the transmission path.

Shown in FIG. 1 is a simplified CDMA communication system that involvesa single communication channel of a given bandwidth which is mixed by aspreading code which repeats a predetermined pattern generated by apseudo-noise (pn) sequence generator. A data signal is modulated withthe pn sequence to produce digital spread spectrum signal. A carriersignal is modulated with the digital spread spectrum signal to establisha forward link and is then transmitted. A receiver demodulates thetransmission to extract the digital spread spectrum signal. The sameprocess is repeated to establish a reverse link.

During terrestrial communication, a transmitted signal is typicallydisturbed by reflections due to varying terrain and environmentalconditions and man-made obstructions. Thus, a single transmitted signalproduces a plurality of received signals with differing time delays atthe receiver, an effect which is commonly known as multipath distortion.During multipath distortion, the signal from each different path arrivesdelayed at the receiver with a unique amplitude and carrier phase.

In the prior art, the error associated with multipath distortion istypically corrected at the receiver after the signal has been correlatedwith the matching pn sequence and the transmitted data has beenreproduced. Thus, the correlation is completed with error incorporatedin the signal. Similar multipath distortion affects the reverse linktransmission.

Accordingly, there exists a need for a system that corrects a signal forerrors encountered during transmission.

SUMMARY

The present invention is an apparatus for reducing transmission errorsby a base station when transmitting a communication signal. An RFcommunication signal is received at the base station and analyzed forerrors. A correction signal is then generated based upon the analysis.Prior to transmission of the communication signal from the base station,the communication signal is corrected using the correction signal.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a simplified block diagram of a prior art CDMA communicationsystem.

FIG. 2 is a detailed block diagram of a B-CDMA™ communication system.

FIG. 3A is a detailed block diagram of the present invention using onepseudo-pilot signal, with carrier-offset correction implemented at thechip level.

FIG. 3B is a block diagram of a rake receiver.

FIG. 4 is a diagram of a received symbol p_(o) on the QPSK constellationshowing a hard decision.

FIG. 5 is a diagram of the angle of correction corresponding to theassigned symbol.

FIG. 6 is a diagram of the resultant symbol error after applying thecorrection corresponding to the assigned symbol.

FIG. 7 is a block diagram of a conventional phase-locked loop.

FIG. 8A is a simple block diagram of a transmitter in accordance withthe preferred embodiment of the present invention.

FIG. 8B is a simple block diagram of a transmitter in accordance with analternative embodiment of the present invention.

FIG. 8C is a simple block diagram of a transmitter in accordance with analternative embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The preferred embodiment will be described with reference to the drawingfigures where like numerals represent like elements throughout.

A CDMA communication system 25 as shown in FIG. 2 includes a transmitter27 and a receiver 29, which may reside in either a base station or amobile user receiver. The transmitter 27 includes a signal processor 31which encodes voice and nonvoice signals 33 into data at various rates,e.g. data rates of 8 kbps, 16 kbps, 32 kbps, or 64 kbps. The signalprocessor 31 selects a specific data rate depending upon the type ofsignal, or in response to a set data rate.

By way of background, two steps are involved in the generation of atransmitted signal in a multiple access environment. First, the inputdata 33 which can be considered a bi-phase modulated signal is encodedusing forward error-correction (FEC) coding 35. For example, if a R=½convolution code is used, the single bi-phase modulated data signalbecomes bivariate or two bi-phase modulated signals. One signal isdesignated the in-phase (I) channel 41 a. The other signal is designatedthe quadrature (Q) channel 41 b. A complex number is in the form a+bj,where a and b are real numbers and j²=−1. Bi-phase modulated I and Qsignals are usually referred to as quadrature phase shift keying (QPSK).In the preferred embodiment, the tap generator polynomials for aconstraint length of K=7 and a convolutional code rate of R=½ areG₁=171₈37 and G₂=133₈39.

In the second step, the two bi-phase modulated data or symbols 41 a, 41b are spread with a complex pseudo-noise (pn) sequence. The resulting I45 a and Q 45 b spread signals are combined 53 with other spread signals(channels) having different spreading codes, mixed with a carrier signal51 and then transmitted 55. The transmission 55 may contain a pluralityof individual channels having different data rates.

The receiver 29 includes a demodulator 57 a, 57 b which downconverts thetransmitted broadband signal 55 into an intermediate frequency signal 59a, 59 b. A second downconversion reduces the signal to baseband. TheQPSK signal is then filtered 61 and mixed 63 a, 63 b with the locallygenerated complex pn sequence 43 a, 43 b which matches the conjugate ofthe transmitted complex code. Only the original waveforms which werespread by the same code at the transmitter 27 will be effectivelydespread. Others will appear as noise to the receiver 29. The data 65 a,65 b is then passed onto a signal processor 67 where FEC decoding isperformed on the convolutionally encoded data.

When the signal is received and demodulated, the baseband signal is atthe chip level. Both the I and Q components of the signal are despreadusing the conjugate of the pn sequence used during spreading, returningthe signal to the symbol level. However, due to carrier offset, phasecorruption experienced during transmission manifests itself bydistorting the individual chip waveforms. If carrier offset correctionis performed at the chip level overall accuracy increases due to theinherent resolution of the chip-level signal. Carrier offset correctionmay also be performed at the symbol level but with less overallaccuracy. However, since the symbol rate is much less than the chiprate, a lower overall processing speed is required when the correctionis done at the symbol level.

As shown in FIG. 3A, a receiver using the system 75 and method of thepresent invention is shown. A complex baseband digital spread spectrumsignal 77 comprised of in-phase and quadrature phase components is inputand filtered using an adaptive matched filter (AMF) 79 or other adaptivefiltering means. The AMF 79 is a transversal filter (finite impulseresponse) which uses filter coefficients 81 to overlay delayed replicasof the received signal 77 onto each other to provide a filtered signaloutput 83 having an increased signal-to-noise ratio (SNR). The output 83of the AMF 79 is coupled to a plurality of channel despreaders 85 ₁, 85₂, 85 _(n) and a pilot despreader 87. The pilot signal 89 is despreadwith a separate despreader 87 and pn sequence 91 contemporaneous withthe transmitted data 77 assigned to channels which are despread 85 ₁, 85₂, 85 _(n) with pn sequences 93 ₁, 93 ₂, 93 _(n) of their own. After thedata channels are despread 85 ₁, 85 ₂, 85 _(n), the data bit streams 95₁, 95 ₂, 95 _(n) are coupled to Viterbi decoders 97 ₁, 97 ₂, 97 _(n) andoutput 99 ₁, 99 ₂, 99 _(n).

The filter coefficients 81, or weights, used in adjusting the AMF 79 areobtained by the demodulation of the individual multipath propagationpaths. This operation is performed by a rake receiver 101. The use of arake receiver 101 to compensate for multipath distortion is well knownto those skilled in the communication arts.

As shown in FIG. 3B, the rake receiver 101 consists of a parallelcombination of path demodulators “fingers” 103 ₀, 103 ₁, 103 ₂, 103 _(n)which demodulate a particular multipath component. The pilot sequencetracking loop of a particular demodulator is initiated by the timingestimation of a given path as determined by a pn sequence 105. In theprior art, a pilot signal is used for despreading the individual signalsof the rake. In the present invention, the pn sequence 105 may belong toany channel 93 ₁ of the communication system. Typically, the channelwith the largest received signal is used.

Each path demodulator includes a complex mixer 107 ₀, 107 ₁, 107 ₂, 107_(n), and summer and latch 109 ₀, 109 ₁, 109 ₂, 109 _(n). For each rakeelement, the pn sequence 105 is delayed τ 111 ₁, 111 ₂, 111 _(n) by onechip and mixed 107 ₁, 107 ₂, 107 _(n) with the baseband spread spectrumsignal 113 thereby despreading each signal. Each multiplication productis input into an accumulator 109 ₀, 109 ₁, 109 ₂, 109 _(n) where it isadded to the previous product and latched out after the nextsymbol-clock cycle. The rake receiver 101 provides relative path valuesfor each multipath component. The plurality of n-dimension outputs 115₀, 115 ₁, 115 ₂, 115 _(n) provide estimates of the sampled channelimpulse response that contain a relative phase error of either 0°, 90°,180°, or 270°.

Referring back to FIG. 3A, the plurality of outputs from the rakereceiver are coupled to an n-dimensional complex mixer 117. Mixed witheach rake receiver 101 output 115 is a correction to remove the relativephase error contained in the rake output.

A pilot signal is also a complex QPSK signal, but with the quadraturecomponent set at zero. The error correction 119 signal of the presentinvention is derived from the despread channel 95 ₁ by first performinga hard decision 121 on each of the symbols of the despread signal 95 ₁.A hard decision processor 121 determines the QPSK constellation positionthat is closest to the despread symbol value.

As shown in FIG. 4, the Euclidean distance processor compares a receivedsymbol p_(o) of channel 1 to the four QPSK constellation points x_(1,1),x_(−1,1), x_(−1,−1), x_(1,−1). It is necessary to examine each receivedsymbol p_(o) due to corruption during transmission 55 by noise anddistortion, whether multipath or radio frequency. The hard decisionprocessor 121 computes the four distances d₁, d₂, d₃, d₄ to eachquadrant from the received symbol p_(o) and chooses the shortestdistance d₂ and assigns that symbol location x_(−1,1). The originalsymbol coordinates p_(o) are discarded.

Referring back to FIG. 3A, after undergoing each hard symbol decision121, the complex conjugates 123 for each symbol output 125 aredetermined. A complex conjugate is one of a pair of complex numbers withidentical real parts and with imaginary parts differing only in sign. Asshown in FIG. 5, a symbol is demodulated or de-rotated by firstdetermining the complex conjugate of the assigned symbol coordinatesx_(−1,−1), forming the correction signal 119 which is used to remove therelative phase error contained in the rake output. Thus, the rake outputis effectively de-rotated by the angle associated with the harddecision, removing the relative phase error. This operation effectivelyprovides a rake that is driven by a pilot signal, but without anabsolute phase reference.

Referring back to FIG. 3A, the output 119 from the complex conjugate 123is coupled to a complex n-dimensional mixer 117 where each output of therake receiver 101 is mixed with the correction signal 119. The resultingproducts 127 are noisy estimates of the channel impulse response p₁ asshown in FIG. 6. The error shown in FIG. 6 is indicated by a radiandistance of π/6 from the in-phase axis.

Referring back to FIG. 3A, the outputs 115 of the complex n-dimensionalchannel mixer 117 are coupled to an n-dimensional estimator 131. Thechannel estimator 131 is a plurality of low-pass filters, each forfiltering a multipath component. The outputs 81 of the n-dimensionalestimator 131 are coupled to the AMF 79. These outputs 81 act as the AMF79 filter weights. The AMF 79 filters the baseband signal to compensatefor channel distortion due to multipath without requiring a largemagnitude pilot signal.

The rake receiver 101 is used in conjunction with the phase-locked loop(PLL) 133 circuits to remove carrier offset. Carrier offset occurs as aresult of transmitter/receiver component mismatches and other RFdistortion. The present invention 75 uses a low level pilot signal 135which is produced by despreading 87 the pilot from the baseband signal77 with a pilot pn sequence 91. The pilot signal is coupled to a singleinput PLL 133, shown in FIG. 7. The PLL 133 measures the phasedifference between the pilot signal 135 and a reference phase of 0. Thedespread pilot signal 135 is the actual error signal coupled to the PLL133.

The PLL 133 includes an arctangent analyzer 136, complex filter 137, anintegrator 139 and a phase-to-complex-number converter 141. The pilotsignal 135 is the error signal input to the PLL 133 and is coupled tothe complex filter 137. The complex filter 137 includes two gain stages,an integrator 145 and a summer 147. The output from the complex filter137 is coupled to the integrator 139. The integral of frequency isphase, which is output 140 to the converter 141. The phase output 140 iscoupled to a converter 141 which converts the phase signal into acomplex signal for mixing 151 with the baseband signal 77. Since theupstream operations are commutative, the output 149 of the PLL 133 isalso the feedback loop into the system 75.

The correction signal 119 of the complex conjugate 123 and the outputsignal 149 of the PLL 133 are each coupled to mixers located within thetransmitter 181, in order to correct the signal before transmission asshown in FIG. 8A. The transmitter 181 shown in FIG. 8A operates in asimilar manner to the transmitter 27 shown in FIG. 2, except that thesignal ready for transmission is pre-rotated prior to transmission.Referring to FIG. 8A, data 164 ₁, 164 ₂, 164 ₃ is encoded using forwardcorrecting coding (FEC) 35. The two bi-phase modulated data or symbols41 a, 41 b are spread with a complex pseudo-noise (pn) sequence and theresulting I 45 a and Q 45 b spread signals are mixed with the correctionsignal 119, upconverted with the carrier signal 51, and combined 53 withother spread signals having different spreading codes. The resultingsignal 55 is again corrected using the signal 149 from the receiver PLL133. The signal 56 which has been pre-corrected for phase and frequencyis then transmitted. In this manner, the present invention utilizes thesignals 119, 149 generated by the receiver 71 to pre-correct thetransmitted signal and reduce the phase and frequency errors in thesignals as received at the receiving unit.

Referring to FIG. 8B, a transmitter 183 made in accordance with analternative embodiment of the present invention is shown. Thisembodiment is similar to the embodiment shown in FIG. 8A, except thatthe correction signal 119 is mixed with the baseband data signal via amixer 157. Thus, the baseband data is pre-corrected prior to encodingand spreading. Of course, those of skill in the art should realize thatother processing steps may be introduced before the correction signal119 is mixed with the data signal.

Referring to FIG. 8C, a transmitter 188 made in accordance with anotheralternative embodiment of the present invention is shown. In thisembodiment, the correction signal 119 and the carrier offset signal 149are input into a combiner, which combines the signal into a singlepre-correction signal, and mixed using the mixer 169 with the output ofthe summer 53 prior to transmission.

Finally, it should be noted that the carrier offset correction and thepre-rotation correction are separate corrections. Each may be utilizedindependently of the other. For example, the system may pre-correct onlyfor carrier offset error and may not perform pre-rotation.Alternatively, the system may perform pre-rotation but may not correctfor carrier offset error.

While specific embodiments of the present invention have been shown anddescribed, many modifications and variations could be made by oneskilled in the art without departing from the spirit and scope of theinvention. The above description serves to illustrate and not limit theparticular form in any way.

What is claimed is:
 1. A base station for transmitting and receivingcommunication signals, comprising a receiver and transmitter, forreducing transmission errors in said transmitted signal, the receiverincluding: an antenna for receiving a communication signal; an adaptivematched filter for filtering said received signal to generate a filteredsignal using a weighting signal; an analyzer for analyzing said receivedsignal for errors and generating a correction signal based on saidanalysis, and said analyzer further includes: at least one despreaderfor despreading said filtered signal using a pilot signal; a processorfor performing a hard decision on said despread filtered signal andgenerating symbol outputs therefrom; and a conjugator for determiningthe complex conjugates of said symbol outputs to generate saidcorrection signal; a correction unit for correcting said received signalusing said correction signal; and whereby said transmitter is responsiveto said correction unit, for correcting said transmitted signal prior totransmission.
 2. The base station of claim 1 wherein said correctionunit comprises: a RAKE receiver for demodulating said received signaland generating relative path values for each multipath component of saidreceived signal; and a mixer for mixing said path values with saidcorrection signal to generate said weighting signal.